Ion implantation is a standard technique for introducing conductivity-altering dopant materials into semiconductor wafers. In a conventional (beam-line type) ion implantation system, a desired dopant material is ionized in an ion source, the ions are accelerated to form an ion beam of a prescribed energy, and the ion beam is directed at a semiconductor wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material.
Compared with the traditional beam-line implanters which have low throughputs at low energies, plasma immersion implant technology is very attractive because it offers the potential of very high throughputs at relatively low implant energies. With the plasma immersion implant technology, an active plasma is typically created with an appropriate gas species, from which positive ions are extracted and implanted into a substrate with the application of direct current (DC) or radio frequency (RF) pulses.
For plasma immersion ion implantation to work, it is necessary to select gas species that, when subjected to either pulsed or continuous plasma discharge, produce desired ions. Gas species suitable for plasma immersion ion implantation typically have the following properties: (1) they are easy to store, either as a gas or in a solid or liquid source with a reasonable vapor pressure; (2) they can be handled by standard gas delivery systems; and (3) they are easily ionizable in a plasma. In addition, the ions produced by the plasma should be able to provide desired results for the specific application targeted, e.g. n-type or p-type implants, gate oxide nitridation, contact resistance reduction, hardening etc.
It is also desirable to prevent unwanted ions from co-implanting into the target wafer. For the specific application of p-type doping where fluorine atoms must be avoided, diborane (B2H6) is a commonly used gas species. Due to safety and stability problems, diborane cannot be provided in its pure state. Rather, diborane is typically diluted in helium or hydrogen. When such diluted diborane is used for plasma immersion ion implantation, the carrier (diluent) gas molecules can become ionized and get implanted in addition to the boron-bearing species, thereby adversely impacting the process by complicating dosimetry, introducing unwanted contaminants, and imposing process space limitations on the implant process.
Use of higher boranes such as octaborane and decaborane has been proposed and implemented in beam-line ion implantation systems. For these systems, the main advantage of higher boranes is the ability to achieve a specific implant depth with a relatively high beam energy. This is considered useful because beam-line implanters have higher beam currents (and therefore higher throughputs) at higher energies than they do at lower energies.
In view of the foregoing, it would be desirable to provide a solution for ion implantation which overcomes the above-described inadequacies and shortcomings.